The invention relates generally to exhaust sensors for automotive applications. Particularly, the present invention relates to an exhaust sensor having a high conductivity co-fire zirconia body.
The automotive industry has used exhaust sensors in automotive vehicles for many years to sense the oxygen content of exhaust gases. For example, sensors are used for alteration and optimization of the air to fuel ratio for combustion.
For oxygen sensing, solid electrolyte sensors are used to measure oxygen partial pressure differences between an unknown gas sample and a known gas sample. A sensor typically has a conductive solid electrolyte between porous electrodes. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas (i.e., reference gas) is usually atmospheric air because oxygen content in the air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (emf) is developed between the electrodes according to the Nerst equation:   E  =            (                        R          ⁢                      xe2x80x83                    ⁢          T                          4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
For proper sensor operation, interfacial impedance of the sensor""s electrochemical cell should be maintained within an effective temperature range which is dependent upon the cell""s composition. With prior art cell formulations, some would often have excessively high electrode/electrolyte interfacial impedance after being kiln fired (firing). The cause of the poor impedance performance of the cells is related to the impurity content of the cell materials, cleanliness in manufacture, and sintering temperature.
Low electrochemical cell impedance is achievable with a variety of co-synthesized yttria stabilized zirconia bodies currently available or described in literature. Some of these have demonstrated a high ionic conductivity, high microstructural homogeneity, and good low temperature stability. However, these materials are incapable of being laminated or otherwise joined to an alumina body in the green (unfired) stage followed by a firing to a high density level (theoretical) of about 93% or higher. These materials typically fail by a crack or separation at or associated with the alumina body/zirconia body interface because of sintering shrinkage and/or thermal expansion mismatch in the firing manufacturing process. Some prior art zirconia body formulations, however, are compatible with some high alumina body formulations. This is primarily because these zirconia body formulations convert about 20 weight % to a monoclinic phase from a tetragonal phase while cooling from the firing process between about 650xc2x0 C. and about 350xc2x0 C. However, these formulations, as stated above, cannot be made into an electrochemical cell with low, stable, electrode impedance.
The deficiencies of the above-discussed prior art are overcome or alleviated by the method of manufacturing a zirconia-alumina body, and a sensor of the present invention. The method of manufacturing the zirconia-alumina body comprises: mixing zirconia, yttria, and alumina with at least one solvent to form a mixture; drying said mixture; laminating said dried mixture to an unfired alumina surface; and co-firing to form the zirconia-alumina body, wherein said zirconia-alumina body comprises about 1 weight % to about 45 weight % monoclinic phase zirconia, based upon the total weight of the zirconia.
The method of making the sensor comprises: mixing zirconia, yttria, and alumina with at least one solvent and at least one dispersant to form a mixture; drying said mixture to form an unfired zirconia body; disposing an electrode on each side of said unfired zirconia body; connecting each electrode to an electrical lead; disposing said unfired zirconia body adjacent to an unfired alumina surface to form an unfired zirconia-alumina body, wherein one of said electrodes is disposed between said zirconia body and said alumina body; and co-firing to form the sensor, wherein the co-fired zirconia-alumina body comprises about 1 weight % to about 45 weight % monoclinic phase zirconia, based upon the total weight of the zirconia.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.